It is often necessary to apply frequency modulation to a voltage tuned oscillator in order to generate frequency modulation. One problem that frequently arises is that as the oscillator frequency is varied the frequency modulation sensitivity of the oscillator also alters, i.e. the tuning sensitivity in Hz/V alters. The variation in sensitivity is dependent on the characteristics of a voltage tuning element, usually a varactor diode, and the value of other components in the oscillator. The variation is usually stable for a given oscillator but tends to be different to a greater or lesser extent from one oscillator to another.
A known solution to this problem is to use a digital to analogue converter to vary the amount of modulation signal applied to the oscillator as the VCO frequency is changed. The system however needs to be calibrated with the aid of a modulation meter and the results stored in a digital form (e.g. look up table) so that the system controlling the D-A converter can correct for the tuning sensitivity variation.